1. Field of the Invention
This invention relates generally to surface acoustic wave (SAW) devices, and more particularly to the elimination or minimization of bulk waves in such devices.
2. Description of the Prior Art
Surface acoustic wave devices, including convolvers and filters, may have bulk modes induced in addition to the desired surface wave. Such bulk modes produce spurious responses which denigrate the usefulness of these devices.
Suppression of bulk modes or waves in surface acoustic wave devices has been the object of several prior art structures and methods.
U.S. Pat. No. 3,887,887 of R. S. Wagers et al describes having the bottom or back surface of a SAW device topographically deformed with indentations, protrusions, channels, etc. The deformations are preferably randomly spaced because the so-called bulk mode is said to be actually a combination of modes or waves which are coherent. Scattering of these modes by these randomly spaced deformations thereby substantially eliminates the spurious signals which are otherwise produced by the bulk modes.
In the Proceedings of the Ultrasonics Symposium of 1977, IEEE Cat. #77CH1264-1SU, an article by A. J. Slobodnik, Jr. et al, entitled low Sidelobe SAW Filters Using Overlap and Withdrawal Weighted Transducers, discloses cutting a deep notch or wedge in the back surface of the substrate crystal.
R. S. Wagers' article in the Proceedings of the IEEE, Vol. 64, No. 5, in May, 1976, entitled Spurious Acoustic Responses in SAW Devices, describes backsurface roughening for bulk mode suppression and gives experimental results.
Bulk Wave Generation by the IDT, by R. F. Milsom, published in Wave Electronics, 2 (1976) proposes a substrate orientation be used which will minimize bulk waves, and refers to the use of absorbent material on the ends and backface of the substrate, but makes no mention of impedance matching of the absorber.
N. Wakatsuki et al have an article entitled Suppression of Bulk Wave in SAW Filter by Metal Over-Lay Design published in the 1976 Ultrasonics Symposium Proceedings, IEEE Cat. #76 CH1120-5SU. Reference is made therein that bulk wave signals radiated at an angle larger than a few degrees to the surface can be attenuated by roughening the bottom surface or applying an acoustic absorber to it.
Acoustic Properties of Tungsten-Vinyl Composites, by S. Lees et al, published in the IEEE Transactions of Sonics and Ultrasonics, Vol. SU-20, No. 1, January 1973, describes a method of fabricating materials with controllable characteristics such as specific acoustic impedance for bulk mode tranducers. Vinyl plastic in powdered form and tungsten powder are used.